1. Field of the Invention
The invention relates to sacrificial agents used in enhanced oil recovery techniques, to methods of producing such sacrificial agents and to enhanced oil recovery techniques employing such agents.
2. Description of the Prior Art
At best only about 50% of the oil in a subterranean reservoir can be drawn to the surface by conventional methods (sometimes referred to as primary recovery methods because they use the natural energy of an oil reservoir to force the oil up through the well to the surface). However, enhanced oil recovery techniques (sometimes referred to as secondary and tertiary recovery methods) make it possible to obtain up to half as much oil again from existing wells. One such enhanced method is a surfactant flooding technique which involves forcing water containing a surfactant from an input well through the oil-bearing rock formation to a production well. The surfactant makes the oil more miscible with the water so that oil displacement is easier to achieve. The surfactant flooding technique holds considerable promise provided that the cost of the surfactant used to release the oil from the substrata allows for a reasonable profit on the recovered oil. At present, the surfactant is the single most costly item in the economics of the surfactant-based oil recovery system.
The amount of surfactant required in such recovery systems is quite large because a large proportion is adsorbed on the subterranean formation and hence is unavailable for oil mobilization. These losses can be reduced if the sites on the oil bearing rock which normally attract the surfactants are treated with low cost so-called sacrificial agents which prevent the surfactant from being adsorbed. Consequently most or all of the surfactant then becomes available for oil mobilization.
Attempts to explain sacrificial activity on the basis of chemical structures have had little success, but physical factors, such as molecular weight and possibly the ability to form electrically charged micellar solutions, may be important. Moreover, some so-called sacrificial agents may exert their effect by acting themselves as surfactants, so the term "sacrificial agents" as used herein is intended to include all such low cost agents which can reduce the amount of conventional surfactant used in the surfactant flooding technique, irrespective of the manner in which they work.
Certain alkaline inorganic salts, e.g. sodium phosphates and sodium silicates, and certain organic materials, e.g. lignosulphonates and modified lignosulphonates, have been suggested for use as sacrificial agents (see, for example, U.S. Pat. No.3,688,844 issued on Sept. 5, 1972 to W. O. Roszelle; U.S. Pat. No. 4,133,385 issued to Texaco Inc. on Jan. 9, 1979; U.S. Pat. No. 4,142,582 issued to Texaco Inc. on Mar. 6, 1979; U.S. Pat. No. 4,157,115 issued to Texaco Inc. on June 5, 1979; U.S. Pat. No. 4,235,290 issued to Texaco Inc. on Nov. 25, 1980; U.S. Pat. No. 4,236,579 issued to Texaco Inc. on Dec. 2, 1980).
The organic sacrificial agents appear to be more cost effective than the inorganic materials and they are generally derived from spent sulphite liquor generated as a by-product during the pulping of wood using the sulphite cooking process.
Other organic sacrificial agents referred to in the prior art are sulphonated alkali lignins (kraft black liquor lignin sold under the trade mark Indulin; see U.S. Pat. No. 4,235,290 mentioned above).
The effectiveness of lignosulphonates as sacrificial agents has spurred investigation of other pulping industry by-products for use in this area. This research has concentrated mainly on pulp bleach plant effluents as sacrificial agents and E-stage bleach plant effluents have been identified as effective for prevening adsorption of anionic surfactants (sulphonated oil type surface active agents) on clays and sandstones. E-stage liquor is a by-product of the well known process of bleaching wood pulp before it is used for paper-making or the like. The pulp is first treated with chlorine (the so-called C-stage) and then with alkali (the E-stage). During tests, pretreatment of oil bearing cor samples with these forest industry byproducts has increased the amount of oil emulsified and recovered by subsequent flooding with surfactant solution by as much as 58% (see U.S. Pat. No. 4,313,500 issued to the U.S. Department of Energy on Feb. 2, 1982).
The main disadvantage of the use of E-stage bleach plant effluent, however, is the low concentration of active ingredients in the liquid (e.g. the total solids content of typical E-stage liquor is generally 0.2-1% by weight). Thus shipment of the product to distant oil wells or evaporation to increase concentration levels is prohibitively expensive. Another drawback is that the effectiveness of E-stage liquor.decreases dramatically with time e.g. during storage.
Thus there is a need for sacrificial agents which are not only inexpensive, but which are also highly effective, easy to obtain in concentrated form, and which do not deteriorate significantly with time.
An object of the invention is to provide sacrificial agents preferably from forest industry or agricultural sources, e.g. the effluent from wood treatment processes.
Another object of the invention is to provide sacrificial agents derived from E-stage liquor which are in fairly concentrated form so that shipment or evaporation to dryness is fairly economical.
Another object of the invention is to provide sacrificial agents which are of improved effectiveness and are thus economical to use.